In recent years, broadband lines which can communicate large volume data at high speed become popular along with a tide of informatization. A data transmitting device such as a router or a WDM (Wavelength Division Multiplexing) device is used for transmitting the large volume data through the broadband lines. The transmitting device includes a plurality of signal processing boards on which a circuit constituted of a combination of at least one arithmetic element such as a LSI and at least one storage unit such as a memory are mounted. The transmitting devices including these signal processing boards are interconnected through the broadband lines.
Each of the signal processing boards has the circuit in which the arithmetic element, the storage unit and the like are connected with each other through electrical wirings. Along with increase of data volume to be processed, each of the signal processing boards having the circuit is recently required to transmit data at extremely-high throughput. However, these signal processing boards gradually suffer from some problems including cross-talk, generation of high-frequency noise, electrical signal deterioration and the like along with speed-up of data transmission. The same problems come to the surface in the fields of a super computer, a large-scale server and the like.
On the other hand, an optical communication technology which uses an optical carrier wave for transmitting data is well-known. In recent years, an optical waveguide is gradually coming into use as means for transmitting the optical carrier wave from one place to another place. The optical waveguide includes a liner (elongated) core portion and a cladding portion provided on a peripheral surface of the liner core portion so as to surround the liner core portion. The core portion is constituted of a substantially transparent material with respect to light (wavelength) of the optical carrier wave and the cladding portion is constituted of a material having a refractive index lower than the core portion.
In the optical waveguide, light inputted from one end portion of the core portion is transmitted to another end portion of the core portion with refracting at boundary planes between the core portion and the cladding portion. A light emitting element such as a semiconductor laser is provided at a side of the one end portion (input side) of the core portion. A light receiving element such as a photodiode is provided at a side of another end portion (output side) of the core portion. The light inputted by the light emitting element is transmitted through the optical waveguide, and then received by the light receiving element. According to a blink pattern or an intensity pattern of the received light, data communication is carried out.
By using the optical waveguide for the electrical wirings provided in the signal processing board, it is expected that the aforementioned problems due to the electrical wirings are solved, and thereby allowing the signal processing board to transmit data at higher throughput.
As the optical waveguide, a step-index type optical waveguide has been generally used. The step-index type optical waveguide includes a core portion having a constant refractive index and a cladding portion having a constant refractive index lower than the core portion. However, a graded-index type optical waveguide which has a continuously varying refractive index is recently used (suggested).
For example, the following patent document 1 discloses a graded-index type optical waveguide having a refractive index distribution whose cross-sectional distribution of the refractive index concentrically varies. Such refractive index distribution is provided by dispersing a refractive index adjuster into a polymer base forming the core portion and the cladding portion of the optical waveguide. The graded-index type optical waveguide can reduce transmission loss as compared with the step-index type optical waveguide.
In recent years, there is a growing need for developing an optical waveguide having greater capacity (higher throughput), multi-channel and high density. Along with multiplexing and densification channels of the optical waveguide, a pitch between the channels (a pitch of the core portions) becomes narrower. This causes new problems such as an optical cross-talk (unwanted leakage light interference between neighboring channels) and a rounding of a pulse signal (unwanted broadening of a pulse signal).
These aforementioned problems are especially noticeable in the optical waveguide of the type provided with a core portion and a cladding portion in one layer so that a partial refractive index difference in one layer of the optical waveguide is generated.